Dependency management in task scheduling

ABSTRACT

A task is marked as dependent upon a preceding task. The task that is attempted to be taken for execution from a head of a pending task queue that is marked is deferred. The deferred task is removed from the pending task queue and placed in a deferred task queue. The deferred task is reinserted back into the pending task queue for execution upon determining that the preceding tasks are completed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/489,374, filed on Jun. 5, 2012.

FIELD OF THE INVENTION

The present invention relates generally to computers, and moreparticularly, to dependency management in task scheduling.

DESCRIPTION OF THE RELATED ART

In today's society, computer systems are commonplace. Computer systemsmay be found in the workplace, at home, or at school. Computer systemsmay include data storage systems, or disk storage systems, to processand store data. Within various computing environments, computer systemsare generally required to multi-task, meaning that the computer systemscan handle a number of different tasks or processes at the same time.Additionally, the various tasks and processes to be completed may eachhave a different relative priority based on a number of differentfactors. Due to these various factors, challenges arise for increasingthe efficiency of the dependency management in task scheduling withinthe computing environment.

SUMMARY OF THE INVENTION

Given that computer systems are required to multi-task, a schedulingsystem is responsible for allocating resources of the computing systemto perform the scheduled tasks. However, tasks that are dependent uponpreceding tasks generate challenges to the task scheduling operations.Tasks are deferred until all preceding tasks (dependent ornon-dependent) finish executing. As such, a need arises for deferringexecution of tasks dependent on preceding tasks within a task queue andmaintaining a constant time complexity attribute without compromisingthe queuing order between two dependent or two non-dependent tasks.

Accordingly, and in view of the foregoing, various exemplary method,system, and computer program product embodiments for dependencymanagement in task scheduling in a computing environment are provided.In one embodiment, by way of example only, a task is marked as dependentupon a preceding task. The task that is attempted to be taken forexecution from a head of a pending task queue that is marked isdeferred. The deferred task is removed from the pending task queue andplaced in a deferred task queue. The deferred task is reinserted backinto the pending task queue for execution upon determining that thepreceding tasks are completed.

In addition to the foregoing exemplary method embodiment, otherexemplary system and computer product embodiments are provided andsupply related advantages. The foregoing summary has been provided tointroduce a selection of concepts in a simplified form that are furtherdescribed below in the Detailed Description. This Summary is notintended to identify key features or essential features of the claimedsubject matter, nor is it intended to be used as an aid in determiningthe scope of the claimed subject matter. The claimed subject matter isnot limited to implementations that solve any or all disadvantages notedin the background.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict embodiments of the invention and are not therefore to beconsidered to be limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a computing system environmenthaving an example storage device in which aspects of the presentinvention may be realized;

FIG. 2 is a block diagram illustrating a hardware structure of anexemplary data storage system in a computer system in which aspects ofthe present invention may be realized;

FIG. 3 is a flow chart diagram illustrating an exemplary method fordependency management in task scheduling;

FIG. 4 is a flow chart diagram illustrating an exemplary method forobtaining a task for execution;

FIG. 5 is a flow chart diagram illustrating an exemplary method forregaining a deferred task from a deferred task queue; and

FIG. 6 is a flow chart diagram illustrating an exemplary method fordetermining if a dependent task may be executed.

DETAILED DESCRIPTION OF THE DRAWINGS

Throughout the following description and claimed subject matter, thefollowing terminology, pertaining to the illustrated embodiments, isdescribed. A “Pending Task Queue” is intended to refer to a queue thatholds the tasks that are pending execution. The pending task queue isassumed to be fair, meaning the queue maintains a “first come firstserve” policy for scheduling. A “Deferred Task Queue” is intended torefer to a queue that holds tasks for which execution is deferred untilthe tasks dependencies are met (e.g., finished executing). The deferredtask queue is assumed to be fair, meaning the queue maintains a “firstcome first serve” policy for scheduling. A “Running Task List” isintended to refer to a list of running triggers. A “task record” isintended to refer to an entity describing a task. Task records arestored in both the pending task queue and in the running task list. A“Task Execution” is intended to refer to the operation of using dataencapsulated in the task record by a processing procedure. The taskexecution process involves moving a task from the pending task queue tothe running task list. A “Preceding Task” is intended to refer to a task(e.g. task “A”) that precedes another task (e.g., task “B”) if task “A”was created (and added to a pending task queue) before task “B”. A “TaskDeferral” is intended to refer to the act of preventing a queued taskfrom being taken for execution by the dispatcher until all of the tasksthat preceded it complete their execution. A “Maximum Number of RunningTasks” is intended to refer to the maximum number of tasks the systemmay execute simultaneously. This is the maximum size of the running tasklist. A “Creation Stamp” is intended to refer to a task attribute basedon a monotonic non-decreasing function. Each newly created task willreceive a creation stamp of the current system. A “Task Returning” isintended to refer to the process of removing a running task from therunning task list and placing the removed task back in the pending taskqueue. The returning of a task is performed if the task execution endedwith a retry request (e.g. if the execution of the task could not becompleted for some reason). A “Task Regaining” is intended to refer tothe process of removing a deferred task from the deferred task queue andputting it back in the pending task queue. The regaining of a task isperformed when the scheduling mechanism determines that the deferredtask may be taken for execution. A “system” may include a computersystem, a computer processor, a primary storage system, a remote storagesystem or any other component within a computer environment needed tocarry out the claimed invention.

As mentioned above, a scheduling system is responsible for allocatingresources to scheduled tasks. A dispatcher is responsible for de-queuingtasks from a task queue and assists in allocating the appropriateresources to the tasks. A task selection process by the dispatcher mayvary within various computing systems, and may be defined as ascheduling algorithm. A long term scheduling system may be a schedulingsystem in which task execution may be deferred until system resourcesare available. The deferred tasks must be kept in a queue, which in somesystems must be kept in a persistent state. A task may be dependent onthe completion of previous tasks (e.g., the task must not be taken forexecution until all of the tasks that preceded the dependent task havecompleted execution). The management process of such dependencies mayentail a performance penalty on the system as the dependent tasks needto be postponed while other non-dependent tasks are taken for execution.Such processes involve computational penalties of retrieving postponedtasks and manipulating the queuing data structure. Moreover, in the caseof persistent computing systems, additional penalties of load and save(e.g. I/O operations on physical storage devices) are incurred. Thus, inorder for a computing system to maintain a constant time complexitydependency management in task scheduling, the computing system mustsatisfy the following essential requirements. It should be noted thatconstant time complexity means the time required to perform an operationis not dependent upon the number of tasks in the queue. (1) Thedependency management must support dependency of a subset of the taskson preceding tasks within the task queue. (2) The dependency managementmust allow for constant time complexity (e.g., managing and handling thedependent tasks should not entail searching the entire task queue). (3)The dependency management shall not compromise the queuing order betweennon-dependent tasks nor the queuing order between dependent tasks. Inother words, the queuing scheme may only be affected in cases in whichone of the tasks is dependent and the other task is non-dependent. (4)The dependency management shall support retrying of tasks, if needed. Inother words, the mechanisms support scheduling systems that optionallyreturn tasks to the queue after execution. Also, the computing systemshould provide support for persistent task queues (and associated datastructures) due to the constant-time complexity the computing systemprovides.

To address these needs, the mechanisms of the illustrated embodimentsseek to provide a solution allowing for constant time complexitydependency management in task scheduling, satisfying all requirementsdescribed above. The mechanisms enable dependency of a subset of taskson previous tasks within the task queue that are within constant timecomplexity. The mechanisms allow for constant time complexity dependencymanagement in task scheduling without compromising the queuing orderbetween two dependent and/or two non-dependent tasks. In other words,the mechanisms of the illustrated embodiments provide a solution bysatisfying the required elements, as stated above, for allowing constanttime complexity dependency management in task scheduling according tothe following requirements. 1) The mechanisms support dependency of asubset of the tasks on preceding tasks within the task queue. 2) Themechanisms allow dependency management operations to have constant timecomplexity (e.g., handling dependent tasks may not entail searching anentire task queue). 3) The mechanisms do not compromise the queuingorder between non-dependent tasks nor the queuing order betweendependent tasks. In other words, the queuing scheme may only be affectedin cases in which one of the tasks is dependent and the other is not. 4)The mechanisms support task retrying. In one embodiment, the mechanismssupport scheduling systems that optionally return tasks to the queueafter execution and provide support for persistent task queues (andassociated data structures) due to the constant-time complexity.

In one embodiment, the mechanisms of the illustrated embodiment enablethe task scheduling within the basic framework of repeatedly removing anext task record (e.g., the next schedule task waiting for execution)from a pending task queue, placing the task record in a running tasklist, and executing the task record. When the task executionsuccessfully completes, the task record is removed from the running tasklist. Alternatively, a task execution may end with a retry request inwhich the task record is removed from the running task list and returnedto the head of the pending task queue in order to be retried. Moreover,the mechanisms preserve the original order of the pending task queuewhen returning the task record to the head of the pending task queue. Inso doing, the mechanisms of the illustrated embodiments allow dependenttasks queued for execution to be deferred until all of the tasks thatpreceded the dependent tasks complete their execution.

In one embodiment, by way of example only, tasks may either be dependentor non-dependent on preceding tasks. One of the attributes of a newlycreated task is an indication as to whether or not the task is dependenton preceding tasks associated with the new task (e.g., whether or notthe execution of the task must be deferred until all preceding taskscomplete their execution. Each task may be “marked” as a task that isdependent upon all preceding tasks. A task that is not marked asdependent on preceding tasks may be taken for execution, without anyconstraints, other than those imposed by a pending task queuingmechanism. In other words, when a non-dependent task is removed from thepending task queue, the non-dependent task may be immediately taken forexecution. However, when a task that is marked as dependent on precedingtasks is removed from the pending task queue, the mechanisms of theillustrated embodiments verify that all of the tasks that preceded thedependent task have completed execution. If one of the preceding tasksof the marked task has yet to be completed, the dependent task isdeferred until the deferred task may be taken for execution. Thedeferred task is removed from a pending task queue, and thus, a nexttask in the queue may be taken for execution.

After removing the deferred tasks from the pending task queue, thedeferred task records may be placed in a designated queue (e.g., adeferred task queue) until the deferred task records (e.g., “deferredtasks) may finally be taken for execution. The ordering of the tasks inthe deferred task queue is identical to the order of the tasks in thepending task queue (e.g., the pending task queue and the deferred taskqueue are assumed to be fair). A task is removed from the deferred taskqueue once all preceding tasks associated with the task have completedexecution. The deferred task is then removed from the deferred taskqueue and placed at the head of the pending task queue in order to betaken for execution.

In order to achieve constant time complexity, the mechanisms allow fordetermining whether or not each preceding task associated with thedeferred task(s) has completed execution within a constant time.Moreover, the mechanisms further allow for determining whether adeferred task may be returned to the pending task queue. Also, themechanisms of the illustrated embodiment only require the scanning of arunning task list, which has an upper bound threshold constant (e.g.,the maximum number of tasks that the system may execute simultaneously),since the ordering of the tasks in the pending task queue and thedeferred task queue are identical. The scanning of the running task listallows the mechanism to eliminate the need for scanning all pending anddeferred tasks.

In addition, since deferred task records are removed from the pendingtask list, the mechanisms do not compromise the order between twonon-dependent tasks. The mechanisms also maintain the order between twodependent tasks, since the deferred tasks are placed in the deferredtask queue and are only returned to the pending task list once thetask(s) are conclusively taken for execution. Furthermore, retried tasks(e.g., tasks that are returned to the pending task queue from a runninglist) do not compromise the dependency between two dependent tasks,because the original order of the pending task queue is preserved.

Hence, the mechanisms of the illustrated embodiments achieve constantcomplexity dependency mechanism in task scheduling, by satisfying thefollowing comprehensive set of requirements. 1) The marking of tasks bythe mechanisms as dependent on preceding tasks enables creating a subsetof tasks, for which execution may be deferred until the completeexecution of all preceding tasks. (2) The deferral of task operations bythe mechanisms of the illustrated embodiments relies on scanning arunning list, which is bounded by a well-known constant. Thus, deferringtasks and determining whether a deferred task may be taken for executionis performed in constant time complexity, without scanning all pendingand deferred tasks. (3) Removing the deferred tasks from the pendingqueue and placing them in a designated queue ensures that the mechanismswill not compromise the queuing order between two dependent tasks andalso between two non-dependent tasks. (4) Since the order of the pendingtask queue is preserved when a task is retried, the proposed mechanismsupports task retrying.

Turning now to FIG. 1, exemplary architecture 10 of a computing systemenvironment is depicted. The computer system 10 includes centralprocessing unit (CPU) 12, which is connected to communication port 18and memory device 16. The communication port 18 is in communication witha communication network 20. The communication network 20 and storagenetwork may be configured to be in communication with server (hosts) 24and 22 and storage systems, which may include storage devices 14. Thestorage systems may include hard disk drive (HDD) devices, solid-statedevices (SSD) etc., which may be configured in a redundant array ofindependent disks (RAID). The operations as described below may beexecuted on storage device(s) 14, located in system 10 or elsewhere andmay have multiple memory devices 16 working independently and/or inconjunction with other CPU devices 12. Memory device 16 may include suchmemory as electrically erasable programmable read only memory (EEPROM)or a host of related devices. Memory device 16 and storage devices 14are connected to CPU 12 via a signal-bearing medium. In addition, CPU 12is connected through communication port 18 to a communication network20, having an attached plurality of additional computer host systems 24and 22. In addition, memory device 16 and the CPU 12 may be embedded andincluded in each component of the computing system 10. Each storagesystem may also include separate and/or distinct memory devices 16 andCPU 12 that work in conjunction or as a separate memory device 16 and/orCPU 12.

FIG. 2 is an exemplary block diagram 200 showing a hardware structure ofa data storage system in a computer system according to the presentinvention. Host computers 210, 220, 225, are shown, each acting as acentral processing unit for performing data processing as part of a datastorage system 200. The cluster hosts/nodes (physical or virtualdevices), 210, 220, and 225 may be one or more new physical devices orlogical devices to accomplish the purposes of the present invention inthe data storage system 200. In one embodiment, by way of example only,a data storage system 200 may be implemented as IBM® System Storage™DS8000™. A Network connection 260 may be a fibre channel fabric, a fibrechannel point to point link, a fibre channel over ethernet fabric orpoint to point link, a FICON or ESCON I/O interface, any other I/Ointerface type, a wireless network, a wired network, a LAN, a WAN,heterogeneous, homogeneous, public (i.e. the Internet), private, or anycombination thereof. The hosts, 210, 220, and 225 may be local ordistributed among one or more locations and may be equipped with anytype of fabric (or fabric channel) (not shown in FIG. 2) or networkadapter 260 to the storage controller 240, such as Fibre channel, FICON,ESCON, Ethernet, fiber optic, wireless, or coaxial adapters. Datastorage system 200 is accordingly equipped with a suitable fabric (notshown in FIG. 2) or network adapter 260 to communicate. Data storagesystem 200 is depicted in FIG. 2 comprising storage controllers 240 andcluster hosts 210, 220, and 225. The cluster hosts 210, 220, and 225 mayinclude cluster nodes.

To facilitate a clearer understanding of the methods described herein,storage controller 240 is shown in FIG. 2 as a single processing unit,including a microprocessor 242, system memory 243 and nonvolatilestorage (“NVS”) 216, which will be described in more detail below. It isnoted that in some embodiments, storage controller 240 is comprised ofmultiple processing units, each with their own processor complex andsystem memory, and interconnected by a dedicated network within datastorage system 200. Storage 230 (labeled as 230 a, 230 b, and 230 n inFIG. 3) may be comprised of one or more storage devices, such as storagearrays, which are connected to storage controller 240 (by a storagenetwork) with one or more cluster hosts 210, 220, and 225 connected toeach storage controller 240.

In some embodiments, the devices included in storage 230 may beconnected in a loop architecture. Storage controller 240 manages storage230 and facilitates the processing of write and read requests intendedfor storage 230. The system memory 243 of storage controller 240 storesprogram instructions and data, which the processor 242 may access forexecuting functions and method steps of the present invention forexecuting and managing storage 230 as described herein. In oneembodiment, system memory 243 includes, is in association with, or is incommunication with the operation software 250 for performing methods andoperations described herein. As shown in FIG. 2, system memory 243 mayalso include or be in communication with a cache 245 for storage 230,also referred to herein as a “cache memory”, for buffering “write data”and “read data”, which respectively refer to write/read requests andtheir associated data. In one embodiment, cache 245 is allocated in adevice external to system memory 243, yet remains accessible bymicroprocessor 242 and may serve to provide additional security againstdata loss, in addition to carrying out the operations as described inherein.

In some embodiments, cache 245 is implemented with a volatile memory andnon-volatile memory and coupled to microprocessor 242 via a local bus(not shown in FIG. 2) for enhanced performance of data storage system200. The NVS 216 included in data storage controller is accessible bymicroprocessor 242 and serves to provide additional support foroperations and execution of the present invention as described in otherfigures. The NVS 216, may also referred to as a “persistent” cache, or“cache memory” and is implemented with nonvolatile memory that may ormay not utilize external power to retain data stored therein. The NVSmay be stored in and with the cache 245 for any purposes suited toaccomplish the objectives of the present invention. In some embodiments,a backup power source (not shown in FIG. 2), such as a battery, suppliesNVS 216 with sufficient power to retain the data stored therein in caseof power loss to data storage system 200. In certain embodiments, thecapacity of NVS 216 is less than or equal to the total capacity of cache245.

Storage 230 may be physically comprised of one or more storage devices,such as storage arrays. A storage array is a logical grouping ofindividual storage devices, such as a hard disk. In certain embodiments,storage 230 is comprised of a JBOD (Just a Bunch of Disks) array or aRAID (Redundant Array of Independent Disks) array. A collection ofphysical storage arrays may be further combined to form a rank, whichdissociates the physical storage from the logical configuration. Thestorage space in a rank may be allocated into logical volumes, whichdefine the storage location specified in a write/read request.

In one embodiment, by way of example only, the storage system as shownin FIG. 2 may include a logical volume, or simply “volume,” may havedifferent kinds of allocations. Storage 230 a, 230 b and 230 n are shownas ranks in data storage system 200, and are referred to herein as rank230 a, 230 b and 230 n. Ranks may be local to data storage system 200,or may be located at a physically remote location. In other words, alocal storage controller may connect with a remote storage controllerand manage storage at the remote location. Rank 230 a is shownconfigured with two entire volumes, 234 and 236, as well as one partialvolume 232 a. Rank 230 b is shown with another partial volume 232 b.Thus volume 232 is allocated across ranks 230 a and 230 b. Rank 230 n isshown as being fully allocated to volume 238—that is, rank 230 n refersto the entire physical storage for volume 238. From the above examples,it will be appreciated that a rank may be configured to include one ormore partial and/or entire volumes. Volumes and ranks may further bedivided into so-called “tracks,” which represent a fixed block ofstorage. A track is therefore associated with a given volume and may begiven a given rank.

The storage controller 240 may include a task deferral module 255, apending task queue module 257, and a deferred task queue module 259. Thetask deferral module 255, the pending task queue module 257, and thedeferred task queue module 259 may work in conjunction with each andevery component of the storage controller 240, the hosts 210, 220, 225,and storage devices 230. The task deferral module 255, the pending taskqueue module 257, and the deferred task queue module 259 may bestructurally one complete module or may be associated and/or includedwith other individual modules. The task deferral module 255, the pendingtask queue module 257, and the deferred task queue module 259 may alsobe located in the cache 245 or other components.

The storage controller 240 includes a control switch 241 for controllingthe fiber channel protocol to the host computers 210, 220, 225, amicroprocessor 242 for controlling all the storage controller 240, anonvolatile control memory 243 for storing a microprogram (operationsoftware) 250 for controlling the operation of storage controller 240,data for control and each table described later, cache 245 fortemporarily storing (buffering) data, and buffers 244 for assisting thecache 245 to read and write data, a control switch 241 for controlling aprotocol to control data transfer to or from the storage devices 230,task deferral module 255, the pending task queue module 257, and thedeferred task queue module 259, in which information may be set.Multiple buffers 244 may be implemented with the present invention toassist with the operations as described herein. In one embodiment, thecluster hosts/nodes, 210, 220, 225 and the storage controller 240 areconnected through a network adaptor (this could be a fibre channel) 260as an interface i.e., via at least one switch called “fabric.”

In one embodiment, the host computers or one or more physical or virtualdevices, 210, 220, 225 and the storage controller 240 are connectedthrough a network adaptor (this could be a fibre channel) 260 as aninterface i.e., via at least one switch called “fabric.” In oneembodiment, the operation of the system shown in FIG. 2 will bedescribed. The microprocessor 242 may control the memory 243 to storecommand information from the host device (physical or virtual) 210 andinformation for identifying the host device (physical or virtual) 210.The control switch 241, the buffers 244, the cache 245, the operatingsoftware 250, the microprocessor 242, memory 243, NVS 216, task deferralmodule 255, the pending task queue module 257, and the deferred taskqueue module 259 are in communication with each other and may beseparate or one individual component(s). Also, several, if not all ofthe components, such as the operation software 250 may be included withthe memory 243. Each of the components within the devices shown may belinked together and may be in communication with each other for purposessuited to the present invention.

As mentioned above, the task deferral module 255, the pending task queuemodule 257, and the deferred task queue module 259 may also be locatedin the cache 245 or other components. It should be noted that the RAMmodule 259 can be compared to short-term memory and the hard disk to thelong-term memory. In other words, the RAM module 259 is random accessmemory. The RAM (random access memory) is the place in a computer wherethe operating system, application programs, and data in current use arekept so that they can be quickly reached by the computer's processor242. The RAM is much faster to read from and write to than the otherkinds of storage in a computer, the hard disk, floppy disk, and CD-ROM.As such, one or more task deferral module 255, the pending task queuemodule 257, and the deferred task queue module 259 maybe used as needed,based upon the storage architecture and users' preferences.

Turning now to FIG. 3, a flowchart illustrating an exemplary method 300for dependency management in task scheduling, is illustrated. The method300 begins (step 302) by marking a task as dependent upon precedingtasks (step 304). The method 300 may defer the marked task when themarked tasks is attempted to be taken for execution from a head of apending task queue (step 306). It should be noted that all deferredtasks are dependent tasks, but dependent tasks may not be deferredtasks. A dependent task may be deferred if the dependent task, whenneeding to be taken for execution, the mechanisms identifies that one ormore of the dependent task dependencies are not yet met (i.e. all of thepreceding tasks have not finished executing). Only at this time will thetask be deferred.

The deferred task is removed from a pending task queue and placed in adeferred task queue (step 308). The deferred task is reinserted backinto the pending task queue for execution upon determining that thepreceding tasks is completed (step 310). It should be noted that thetasks are retuned back to the pending task queue in a way that preservesthe pending task queue's original order. In other words, the tasks areretuned back to the pending task queue according to the creation stampof the tasks. The method 300 ends (step 312).

To assist with deferring a task for execution by marking the deferredtask as dependent upon preceding tasks, the mechanisms of theillustrated embodiments use a non-decreasing function to label each newtask with a creation stamp. The mechanisms use the non-decreasing(monotonic) function over time to generate the creation stamp. In such afunction f, if time “T₁” is greater than time “T₂” (e.g., T₁>T₂), thenthe function f(T₁) is greater than or equal to f(T₂) (e.g.,f(T₁)≧f(T₂)), where f is the function, T₁ is a first time and T₂ is asecond time period. Hence, the creation stamps generated will neverdecrease over time and the task records in the pending task queue willbe sorted according to the assigned creation stamp for each task. Thisorder is an invariant of the pending task queue, and the schedulingmechanism ensures that the invariant is preserved.

In order to preserve this invariant, only new tasks may be placed at theend of the pending task queue. Existing tasks that need to be returnedto the pending task queue, either from the deferred queue (regaining) orfrom the running list (returning), are always placed at the head of thepending task queue, in a manner that will keep the queue sorted (e.g.,an insertion sort). Such an insertion sort does not compromise theconstant-time complexity requirement of the mechanism.

Also, since the task creation stamp is non-decreasing, determining if atask depends on preceding tasks may be taken for execution may bereduced to determining if a task with a smaller creation stamp exists ineither the pending task queue or in the running task list. This isachieved in constant time because the sorted pending task queue allowsfor comparing only the first element in the pending task queue andbecause the maximum size of the running list is bounded by a constantthreshold, which is the maximum number of running tasks.

The following description of the mechanisms illustrate how the sortingof the tasks in the pending task queue does not compromise theconstant-time complexity requirement. First, a task is regained from thedeferred task queue and returned to the pending task queue only if thereare no existing tasks with a smaller creation stamp in either thepending task queue or the running list. Specifically, the creation stampof the deferred task is less than the creation stamp of the first taskin the pending task queue. Thus, the deferred task may be placed at thehead of the pending task queue without the need for performing aninsertion sort operation. Second, if a task is returned from the runningtask list back to the pending task queue, the depth of the insertionqueue of the returned task into the pending task queue (e.g., the orderin which the task is placed back into the pending task queue) is at mostequal to the amount of the maximum number of running tasks, since tasksare always returned to the head of the queue. Because the maximum numberof running tasks number is constant, the insertion sort operation of thereturned task has constant time complexity.

As will be described below in FIG. 4, the mechanisms illustrate anexemplary method for getting the next task for execution. First, themechanisms try to regain a deferred task from the deferred task queue.The mechanisms then remove the next task record from the pending taskqueue. The task record is now checked to determine whether or not thenext task is dependent on any preceding tasks. If the task is notdependent on any preceding tasks, the task record is placed in therunning task list and the task is executed. If the task is dependent onpreceding tasks, the mechanisms determine whether or not it can be takenfor execution. If the task can be taken for execution, the task recordis placed in the running task list and the task is executed. Otherwise,if the task may not be taken for execution the task is deferred and thetask record is placed at the tail of the deferred task queue.

The procedure for obtaining a task for execution is hereby described byturning now to FIG. 4. FIG. 4 is a flowchart illustrating an exemplarymethod 400 for obtaining a task for execution. The method 400 begins(step 402) by attempting to regain (e.g., acquire) a deferred task froma deferred task queue (step 404). The method 400 may remove the nexttask record (e.g., the next schedule task waiting for execution) from apending task queue (step 406). The method 400 may then determine if thenext task in the pending task queue is dependent upon any precedingtasks (step 408). If no, the method 400 may add the task to a runningtask list and execute the task (step 416). If yes, the method 400 maycheck if the dependent task may be taken for execution (step 410) anddetermine if the task may be executed (step 412). If the task may beexecuted, the method 400 may add the task to a running task list andexecute the task (step 416). The method 400 ends (step 418). If the taskmay not be executed, the method 400 may defer the task for execution byplacing the task record at the end of the deferred task queue (step414), and, once again, the method 400 ends (step 418).

As will be described below in FIG. 5, the mechanisms illustrate anexemplary embodiment for trying to regain a deferred task. Themechanisms start by checking if there are any deferred tasks in adeferred task queue (e.g., determining whether or not a deferred taskqueue is empty). If there are deferred tasks, the first deferred taskrecord in the deferred task queue is retrieved from the deferred taskqueue. The mechanisms determine if the deferred task may be taken forexecution (e.g., determining whether or not each of preceding tasksassociated with the deferred task has finished execution). If a deferredtask may be taken for execution, it is regained, i.e. the task record isplaced at the head of the pending task queue, so that the regained taskwill be the next task to be taken for execution in the pending taskqueue. If the deferred task may not be taken for execution yet, the taskrecord of the deferred task is returned to the head of the deferred taskqueue. The procedure for regaining a deferred task is hereby describedby turning now to FIG. 5.

FIG. 5 is a flowchart illustrating an exemplary method 400 for regaininga deferred task from a deferred task queue. The method 500 begins (step502) by determining if there are any deferred tasks in a deferred taskqueue (step 504). If no, the method 500 ends (step 516). If yes, themethod 500 removes the first task record from the deferred task queue(step 506). The method 500 checks if the dependent task may be taken forexecution (step 508) and determines if the method 500 may execute thetask (step 510). If no, the method 500 returns the task record to thehead of the deferred task queue (step 512). If yes, the method 500regains (e.g., acquires) the task from the deferred task queue and addsthe task record to the head of the pending task queue (step 514). Themethod 500 ends (step 516).

As described above in FIG. 5 (step 508), the mechanisms check if thedependent task may be taken for execution. To assist with the checkingoperation, in one embodiment, the mechanisms first check whether or notthe creation stamp of the deferred task is smaller or equal to that ofthe first task in the pending task queue. This comparison to creationstamps is sufficient for the regaining of the deferred task from thedeferred task queue because the pending task queue has been sortedaccording to the creation stamps. If the creation stamp of the deferredtasks is not less than or equal to that of the first task in the pendingtask queue, the dependent task may not be taken for execution yet. Inother words, preceding tasks associated with the deferred task have notfinished execution. However, if the creation stamp of the deferred taskis less than or equal to that of the first task in the pending taskqueue, the mechanisms also determines if the smallest creation stamp ofa task in the running tasks list is smaller than or equal to thecreation stamp of the deferred task. Moreover, obtaining the minimalcreation stamp (e.g. the smallest creation stamp) may be accomplished inconstant time complexity since the running task list is bound by themaximum number of running tasks. If the smallest creation stamp of thetask in the running tasks list is smaller than or equal to the creationstamp of the deferred task, then the dependent task cannot be taken forexecution. If the smallest creation stamp of the task in the runningtasks list is greater than the creation stamp of the deferred task, thenthe dependent task may be taken for execution.

The procedure for checking if a dependent task may be executed is herebydescribed by turning now to FIG. 6. FIG. 6 is a flowchart illustratingan exemplary method 600 for determining if a dependent task (e.g.,deferred task) may be executed. The method 600 begins (step 602) bydetermining if a creation stamp assigned to a dependent task is lessthan or equal to the first task in a pending task queue (step 604). Ifno, the method 600 may execute the dependent task (step 610). If yes,the method 600 determines if the creation stamp assigned to thedependent task is less than or equal to the minimal creation stamp of atask in a running task list (step 606). If no, again, the method 600 mayexecute the dependent task (step 610). If yes, the method 600 takes thedependent task for execution (step 608). The method 600 ends (step 612).

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wired, optical fiber cable, RF, etc., or any suitable combination of theforegoing. Computer program code for carrying out operations for aspectsof the present invention may be written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Java, Smalltalk, C++ or the like and conventionalprocedural programming languages, such as the “C” programming languageor similar programming languages. The program code may execute entirelyon the user's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention have been described above withreference to flowchart illustrations and/or block diagrams of methods,apparatus (systems) and computer program products according toembodiments of the invention. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks. The computer program instructions may also beloaded onto a computer, other programmable data processing apparatus, orother devices to cause a series of operational steps to be performed onthe computer, other programmable apparatus or other devices to produce acomputer implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the above figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

While one or more embodiments of the present invention have beenillustrated in detail, the skilled artisan will appreciate thatmodifications and adaptations to those embodiments may be made withoutdeparting from the scope of the present invention as set forth in thefollowing claims.

What is claimed is:
 1. A method for dependency management in task scheduling in a computing environment using a processor device, comprising: marking a task as dependent upon preceding tasks, wherein the task that is attempted to be taken for execution from a head of a pending task queue that is marked is deferred; removing the deferred task from a pending task queue and placing in a deferred task queue; and reinserting the deferred task back into the pending task queue for execution upon determining that the preceding tasks is completed.
 2. The method of claim 1, further including, adding to a running tasks list tasks not dependent upon the preceding tasks, wherein the running tasks list includes the tasks that are currently being executed and includes a bounded constant that is the maximum allowable number of simultaneously executing tasks.
 3. The method of claim 2, further including, executing the tasks in the running tasks list, the executed tasks removed from the running tasks list.
 4. The method of claim 3, further including, if at least one of the tasks in the running tasks list is returned to the pending task queue, reinserting the at least one of the tasks into the pending task queue, wherein a creation stamp preserves the at least one of the tasks order in the pending task queue.
 5. The method of claim 1, further including labeling tasks with a creation stamp by using a non-decreasing function calculation to generate the creation stamp, wherein the creation stamp is a task attribute that will not decrease over time.
 6. The method of claim 5, further including sorting tasks in the pending task queue according to the creation stamp assigned for the labeling.
 7. The method of claim 5, further including, in conjunction with the deferring, ordering the deferred task in the deferred task queue according to the creation stamp assigned for the labeling, wherein the order of the deferred task in the deferred task queue is identical to the order of tasks in the pending task queue.
 8. The method of claim 1, further including, in conjunction with determining that each of the preceding tasks are completed, performing at least one of: determining if a creation stamp of the preceding tasks that is first in the pending task queue is greater than the creation stamp of the deferred task, and determining if the creation stamp for a task in a running task list is greater than the creation stamp of the deferred task.
 9. The method of claim 8, further including, performing at least one of: reinserting the deferred task at the head of the pending task queue for execution if the creation stamp of the preceding tasks that is first in the pending task queue is greater than the creation stamp of the deferred task, and reinserting the deferred task at the head of the pending task queue for execution if the creation stamp for the task in a running task list in the running task list is greater than the creation stamp of the deferred task. 